Many clock generation and clock recovery circuits use phase synthesizers to synthesize an arbitrary output clock that is phase controlled by an N-bit digital input word. A phase synthesizer typically consists of a multiplexer that is used to select two clock phases out of multiple input clock phases and a phase interpolator that generates a clock phase between the two selected clock phases. A phase synthesizer is similar to a digital-to-analog converter (DAC) except that the output is a phase of a periodic waveform rather than a specific voltage. Accordingly, phase synthesizers are sometimes referred to as phase D/A converters or phase DACs. As in D/A converters, the nonlinearities in the input-to-output transfer characteristics are of concern and need to be measured and characterized. The current methods used to measure these phase nonlinearities are overly time-consuming and therefore are not suitable for high-volume product screening environments. For example, it takes approximately one hour with an oscilloscope to measure the average phase position for each digital setting of a phase DAC.
There is a need, therefore, for methods and circuitry for shortening the time needed to characterize nonlinearities in phase DACs.
Like reference numerals refer to the same or similar components throughout the several views of the drawings.